1. Field of the Invention
This invention generally relates to the testing of underground formations or reservoirs, and more particularly to an apparatus and method for effecting a cleaned and sealed well borehole wall portion for improved formation fluid sampling from a formation surrounding the wall portion.
2. Description of the Related Art
Formation testing while drilling (“FTWD”) is a form of formation evaluation that incorporates aspects of wireline logging into a drilling operation. Today, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. A large portion of the current drilling activity involves directional drilling, i.e., drilling boreholes deviated from vertical and/or horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from earth formations. Modem directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations. For the purposes of the present invention, the term Formation Testing While Drilling (“FTWD”) includes, but is not necessarily limited to MWD and LWD tests.
Various types of drilling fluids are used to facilitate the drilling process and to maintain a desired hydrostatic pressure in the borehole. Pressurized drilling fluid (commonly known as the “mud” or “drilling mud”) is pumped into a drill pipe through a central bore to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The mud exits the drill string at the drill bit and returns to the surface in the annular space between the drill string and the borehole wall carrying formation debris (“cuttings”) pulverized by the rotating drill bit. The term (“return fluid”) is used herein to mean fluid comprising drilling fluid, formation fluid and cuttings returning to the surface or otherwise existing in the annulus. The terms drilling fluid, mud, clean fluid or the like are used to mean fluid in the drill string and/or fluid in close relation to any exit port of the drill string and substantially free of cuttings. Such clean fluid may be drilling fluid pumped from a surface location or any substantially clean fluid in the tool.
The clean drilling fluid, typically mixed with additives at the surface, is also used to protect downhole components from corrosion, and to maintain a specified density based on known or expected formation pressure. The return fluid in the annulus is typically maintained at a pressure slightly higher than the surrounding formation. The annular pressure is reduced during certain testing operations that require production of formation fluid.
Several FTWD operations involve producing fluid from the reservoir by, for example, sealing a portion of the borehole and collecting samples of fluid from the formation. Well-known devices such as packers, snorkel probes and extendable pads are typically used to effect a seal at the borehole wall thereby separating the annulus into at least two portions, i.e. one portion being a sealed portion containing formation fluid for testing and at least one more annular portion containing mostly return drilling fluid.
Whenever the sealing device fails to maintain a good seal, the sealed portion may become contaminated with return fluid or pressure control within the sealed portion becomes unmanageable due to pressure communication between the sealed portion and the rest of the annulus.
A common cause sealing problems is the existence of cuttings in the return fluid. As a sealing device is moved to engage the borehole wall, cuttings or thick mud layers are trapped between the sealing device and wall or trapped within the sealed portion. In the former instance the seal is poor, thereby allowing leakage across the seal. In the latter instance cuttings debris can clog the sampling tool or otherwise corrupt the test. The cuttings might also become lodged within a sampling port causing damage or loss of sampling capability.
When starting to pump formation fluid through the sealed portion the mud layer is removed first and enters the formation testing device as well as the formation fluid. The mud contaminates the sample and makes the determination of certain formation parameter more difficult or even impossible.